If an accident has happened

Don't touch or move fresh cracks and have them repaired as soon as possible. The repair will turn out nicer if there's no grease or dirt or frayed edges. If a piece breaks off keep it and any splinters you can find in a box and have it re-glued as soon as possible. It will save you money if the violinmaker doesn't have to fit a new piece. Take the stress off the affected area: loosen the strings, don't force a peg in a cracked pegbox etc. Finally, it might be tempting, but don't try home repairs, you can make things worse.